Everything on, phase 2: circle posts for everyone, private posts on signed refetch, browsers past SecureMode

Circles (owner decision 2026-10-04: fix them for compatibility):
- Mastodon 4.7 and GoToSocial drop a post that names none of their accounts, and a circle post named only the circle
  and its /flock. OutboxPublisher.Publish now sends each member a copy that also names that member in `cc`, on the
  activity and on the object, and names no other member. The Create, every Update (edit, poll, quote approval, policy,
  through the new PublishUpdate) and the Delete (StatusService.Remove now uses Publish) all go that way.
- UpdateOf renders with the post's group, so an Update keeps a circle post's `audience` and a community post's `Page`
  and title.
- A reply to a circle post stays in the circle, whichever client wrote it.
- A circle post can no longer quote a post that needs permission: asking would show the circle post to its author.

Posts that are not public, on refetch (SignedFetchAuthorizer.MayRead):
- Followers-only, direct and circle posts are served to a signed request from someone they were for, or from the
  instance actor of a server where one of them lives. That is a follower or an addressed account, an addressed
  account, or a member. Everyone else still gets 404.
- Once deleted they answer those readers 410. Mastodon deletes its copy when a refetch answers 404.
- A circle refetch names the requesting member, or the members on the requesting server, as the delivered copy did.
- /grunts/create-{id} serves the same.
- /peasants/{name}/whispers/{id}, a DM's `context`, was never routed. It is now the conversation's posts, for its
  participants only.

SecureMode lets browsers through to the redirect to the public page, instead of answering them 401.

653 tests pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

FEDERATION.md

@@ -73,8 +73,10 @@ A group is either a **community** or a **circle**.
   top-level post is a `Page` with a `name`. Members are counted at `/flock`; moderators are listed at `/wardens`, which
   the actor's `attributedTo` points to, with `postingRestrictedToMods` as Lemmy expects. A mention of a community posts into it.
 - A **circle** is private. Its actor is not discoverable and every follow is a request. Its posts are addressed to the
-  circle and its members collection, delivered to each member's own inbox and never announced. They are served only
-  to a signed request from a member, or from the instance actor of a member's server; anyone else gets 404. A
+  circle and its members collection, delivered to each member's own inbox and never announced. Each member's copy also
+  names that member in `cc`, so servers that keep only posts naming one of their accounts (Mastodon, GoToSocial) keep
+  it; it names no other member. The posts are served only to a signed request from a member, or from the instance actor
+  of a member's server, and that copy names the member (or the members on that server); anyone else gets 404. A
   Mastodon member's replies reach only the people they mention.
 - Announces from **remote** groups (Lemmy communities) are followed through to the activity: the object is fetched
   from its own origin, never taken from the announce.
@@ -192,6 +194,13 @@ Posts with a location (shown to nearby users of this server) never leave the ser
   deletes, must be on its actor's origin. An embedded object from another origin is fetched from that origin.
 - **Fetching.** All fetches are signed by the instance actor. They go only to public addresses, follow at most three
   redirects and read at most 1 MB.
+- **Reading our documents (SecureMode).** privapub.thepra.dev answers ActivityPub GETs only when they are signed, like
+  Mastodon's authorized fetch; the instance actor `/peasants/privapub` is the exception, since its key is needed first.
+  A browser asking for HTML is redirected to the public page instead.
+- **Posts that are not public** (followers-only, direct, circle) are served to a signed request from someone they were
+  for, or from the instance actor of a server where someone they were for lives, and to nobody else (404). Once deleted
+  they answer those same readers 410. A direct message's `context`, `/peasants/{name}/whispers/{id}`, is an
+  `OrderedCollection` of the conversation's posts for its participants.
 - **HTML.** Received HTML is sanitised to Mastodon's allowlist.
 - **Keys we cannot fetch for now.** When a sender's key cannot be fetched because its server timed out or answered
   5xx, the inbox answers 503 with `Retry-After: 300` rather than 401.